add DEVELOPER makefile knob to check for acknowledged warnings

We assume Git developers have a reasonably modern compiler and recommend
them to enable the DEVELOPER makefile knob to ensure their patches are
clear of all compiler warnings the Git core project cares about.

Enable the DEVELOPER makefile knob in the Travis-CI build.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.travis.yml

@@ -21,12 +21,12 @@ addons:
 
 env:
   global:
+    - DEVELOPER=1
     - P4_VERSION="15.2"
     - GIT_LFS_VERSION="1.1.0"
     - DEFAULT_TEST_TARGET=prove
     - GIT_PROVE_OPTS="--timer --jobs 3 --state=failed,slow,save"
     - GIT_TEST_OPTS="--verbose --tee"
-    - CFLAGS="-g -O2 -Wall -Werror"
     - GIT_TEST_CLONE_2GB=YesPlease
     # t9810 occasionally fails on Travis CI OS X
     # t9816 occasionally fails with "TAP out of sequence errors" on Travis CI OS X

Documentation/CodingGuidelines

@@ -171,6 +171,11 @@ For C programs:
 
  - We try to keep to at most 80 characters per line.
 
+ - As a Git developer we assume you have a reasonably modern compiler
+   and we recommend you to enable the DEVELOPER makefile knob to
+   ensure your patch is clear of all compiler warnings we care about,
+   by e.g. "echo DEVELOPER=1 >>config.mak".
+
  - We try to support a wide range of C compilers to compile Git with,
    including old ones. That means that you should not use C99
    initializers, even if a lot of compilers grok it.

Makefile

@@ -380,6 +380,18 @@ ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)
 STRIP ?= strip
 
+ifdef DEVELOPER
+CFLAGS += -Werror \
+	-Wdeclaration-after-statement \
+	-Wno-format-zero-length \
+	-Wold-style-definition \
+	-Woverflow \
+	-Wpointer-arith \
+	-Wstrict-prototypes \
+	-Wunused \
+	-Wvla
+endif
+
 # Create as necessary, replace existing, make ranlib unneeded.
 ARFLAGS = rcs